What caused the strobe effect on the raindrops during the thunderstorm?

[GENIERE]

I opened the garage doors and turned on a pair of 150-watt incandescent (note INCANDESCENT) floodlights to admire a torrential thunderstorm and newly created river rushing down my driveway.

What amazed me most was a strobe effect on some of the raindrops. There was little or no wind, the droplets seemed to fall at 4 different rates. The slowest, presumably smallest, fell only a few feet per second like very fine snow. The next fell at a rate that one could track continuously as it passed through the floodlight beam. Others seemed to draw a line as they passed through the beam similar to a meteor trail. The fastest drew a DIScontinuous line as they passed through the beam. As they passed at about two meters in front of me, they formed a broken line segment about 2 cm bright, 2 cm invisible, exactly as one would observe if they were illuminated by a strobe lamp. The only illumination was the incandescent flood lamps (no dimmer in circuit). The strobe effect seemed only to occur with the fastest (largest?) droplets.

I can’t think of anything that would cause this.

 

[Danger]

Drugs?[color=#eded]...[/color]

 

[GENIERE]

Drugs?[color=#eded]...[/color]

No thanks.

 

[matthew baird]

Maybe you are seeing different droplets reflecting light to your eyes..maybe a few different droplets appear as one droplet dropping to the ground. Just a theory...

 

[Duncan]

Maybe there is a slight strobe like effect from the incandescent lights due to the alternating current of the power source?

"[URL ]http://www.photo.net/bboard/q-and-a-fetch-msg?msg_id=00Bh5c&tag=

This page seems to support this hypothesis.

 

[DaveC426913]

Link busted (an extra http// in there). Here is the correct link:

http://www.photo.net/bboard/q-and-a-fetch-msg?msg_id=00Bh5c&tag=""

Unfortunately, it does NOT support the strobe hypothesis, since the objects in the SUV picture are not being lit by any strobe source.

 

[DaveC426913]

I put forth the following hypothesis:

The first three size of drops are not doing anything out of the ordinary:
- Small drops are falling slowly like snow,
- Medium drops are falling fast enough to track,
- Larger drops are falling fast enough to be a blur
So, the only thing needing explaining is the fourth type, the discontinuous line.

I submit that the discontinuous trails are a trick of the eye. The eye does not move smoothly, but jerks very rapidly - faster than we are conscious of. This is known to produce stroboscopic effects under the right circumstances.

I'd go further, to suggest that the difference between types I-III and type IV is a matter of what you are trtying to focus on - something moving or something still. I'd bet that you couldn't see types I-III and type IV simultaneously (though you may not realize it).

 

[GENIERE]

…Maybe there is a slight strobe like effect from the incandescent lights due to the alternating current of the power source?

It seems a camera can detect the warming cooling cycle and I have seen he effect on film before but never took note of it. I just now dimmed a halogen lamp using a common residential dimmer control. I dimmed it to the point where it extinguished if dimmed any further. I could easily count the turns in the coiled filament. I could not discern any flickering even if I shook my head rapidly.

…I submit that the discontinuous trails are a trick of the eye. The eye does not move smoothly, but jerks very rapidly - faster than we are conscious of. This is known to produce stroboscopic effects under the right circumstances.

Any links?

 

[Farsight]

Try waving your hand rapidly back and forth in front of the light source, Geniere, and compare with the same in front of the TV in a darkened room.

Oh go on then, I've just run out back and looked at the spray from the hosepipe in torchlight. There did seem to be something of a morse-code effect with some dots and some dashes. Interesting.

 

[GENIERE]

…Try waving your hand rapidly back and forth in front of the light source, Geniere, and compare with the same in front of the TV in a darkened room.

Been doing that since 1946 when my dad got our first TV.

…Oh go on then, I've just run out back and looked at the spray from the hosepipe in torchlight. There did seem to be something of a morse-code effect with some dots and some dashes. Interesting.

Using a garden hose (good idea-thanks) and floodlights I was able to duplicate the strobe effect. It required adjusting the hose nozzle to obtain large droplets. The effect was not quite the same in that the bright/invisible components of the trail were about 4cm rather than 2cm. I attribute that to the droplets moving slower.

My 3 million candlepower flashlight (torchlight?) decided not to work tonight so I tried to use my SUV’s headlights to eliminate the possibility of the effect being caused by 60hz ac power. Using the garden hose and the headlights, I did discern the effect; at least I think I did. I need to find a way to eliminate glare and use a wider part of the beam.

My theory of the moment is that the droplets are oscillating as they fall

 

[DaveC426913]

My theory of the moment is that the droplets are oscillating as they fall

Oscillating how? And how would that cause the effect?

 

[Mk]

Drugs?... ...

 

[GENIERE]

No longer stumped!

No longer stumped!

http://www.highbeam.com/library/docFree.asp?DOCID=1G1:3670324

“Moreover, there is evidence that raindrops can take on other shapes and that they actually oscillate from one shape to another while they fall. Under the right conditions during a heavy, nighttime rainstorm, anyone can see the effects of these oscillations,…Illumination of rain at night reveals scintillations in the light reflected from the falling drops," says Beard, " with the drops often appearing as dashed streaks… Each raindrop behaves like a little falling lens. Light from, say, a floodlight shining against a dark background bends as it enters each droplet and is reflected back out. As the lens oscillates, the light is reflected in different directions. Thus, a stationary observer sees the bright, reflected light only for brief intervals, when the light happens to go in the viewer's direction. This creates the illusion of broken streaks of light.

 

[Farsight]

Good one, Geniere. We learn something new every day!

 

[DaveC426913]

Wow. Talk about getting the answer to your question...It's like they wrote that just for you.